The effects of the glutathione transferases on 1) conjugate production and 2) covalent binding of acetaminophen were assessed in vitro. The addition of the glutathione transferases to microsomal incubations containing acetaminophen increased the amount of GSH conjugate produced and decreased the amount of covalently bound metabolite when compared to boiled enzyme controls. These transferase effects were more pronounced at low GSH concentrations (0.25 mM) than at high GSH concentrations (2.5 mM). The addition of transferase enzymes to incubations containing either cysteine or N-acetylcysteine in place of glutathione, had little effect on either conjugate production or covalent binding when compared to boiled enzyme controls.